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Belief Revision: The Resubsumption Theory 341

not contradictions between beliefs on the one hand, and data, observations
or experiences, on the other. Instead, a cognitive conflict is a three-way rela-
tion between a background theory that subsumes two contending theories for
some domain, where “theory” refers to informal, perhaps implicitly held belief
systems. A background theory is itself a belief system and hence changeable.
This analysis of cognitive conflict implies that there are three ways for a
conflict to be resolved. Local coherence can be restored by changing the truth
value of the contender theory to false; this is resistance. Alternatively, it can
be restored by altering the truth value of the resident theory; this is conver-
sion. The third possibility is to revise the background theory in such a way that
the two theories are seen as compatible. For example, we do not believe that a
person can be both a human and a wolf, because in the post-genomic era, we
do not believe that living tissue can morph drastically in a short period of time.
In medieval times, thinking with a different background theory, we shuttered
our windows against werewolfs. 17
The third type of resolution is clearly visible in science. One of the pecu-
liarities of quantum physics is that it does not assert material incompatibili-
ties where 19th-century physicists expected them: A particle can have both a
certain spin and its opposite; a radioactive atom can both have and have not
decayed; photons can behave like both particles and waves; and so on. Physicists
have proven that these compatibilities are real by exploiting them for practical
purposes, as in quantum computers. In this case, the incompatibilities were
resolved by revising the relevant background theory to say that something can
be both; for example, quantum physicists found a mathematical representa-
tion that has both a particle and a wave description of elementary particles as
special cases. Abandoning the commonsense background theory that implied
these incompatibilities caused a major philosophical debate among physicists
and philosophers of science about the nature of physics in the first half of the
20th century. Similarly, some Christian biologists try to reconcile their reli-
18
gious beliefs with their scientific practices by saying, for example, that natural
selection is God’s way of creating biological species. In this case the relevant
background theory is the traditional reading of the Book of Genesis in the
Bible as referring to instantaneous creation. The philosophical heartaches
19
that cases like these have caused Western intellectuals indicates that this third
type of conflict resolution is uncommon.
A theory of belief formation should explain the origin of cognitive
conflicts. Why, how and when does the type of mental state that requires a
choice between incompatible beliefs arise? In particular, a satisfactory theory
must explain why nascent conflicts are not dealt with through the resistance